1. Field of the Invention
The present invention relates to a method for synthesis of an oxime by ammoximation of a carbonyl compound containing 6 to 20 C atoms.
2. Discussion of the Background
European Patent Applications 0208311, 0496385, 0299430 and 0564040 as well as U.S. Pat. No. 4,745,221 teach the ammoximation of carbonyl compounds, especially alkanones and cycloalkanones, using hydrogen peroxide and ammonia in the presence of a heterogeneous catalyst composed of the elements silicon, titanium and oxygen. Complete conversions and corresponding simple workup and recovery of the organic solvent used are described only for the ammoximation of cyclohexanone. As is known, cyclohexanone oxime is the raw material for caprolactam synthesized by the Beckmann rearrangement.
During the ammoximation of sterically bulky ketones such as alkanones or cycloalkanones containing more than 6 carbon atoms, the yields are generally relatively low, reflecting the relatively low reaction rates and relatively low hydrogen peroxide selectivities. To overcome these disadvantages it has been proposed that the catalyst system be supplemented by further components, to be referred to as cocatalysts hereinafter. For example, amorphous silicates have been described as cocatalysts in German Patent 19521011, as have acid solids in unexamined German patent application 10047435 and ammonium ions in unexamined German patent application 10103581.
As described in unexamined German patent applications 10047435 and 10103581, the ammoximation of large and sterically shielded alkanones and cycloalkanones becomes sufficiently fast and selective if the reaction is performed in the presence of a suspension catalyst and of an organic solvent that is completely or partly miscible with water, especially a short-chain alcohol containing 1 to 6 carbon atoms.
In the described ammoximation, the alkanone or cycloalkanone used reacts with ammonia and hydrogen peroxide on titanium silicalite to form the corresponding oxime.
In addition to the ammoximation, secondary oxidation reactions between hydroxylamine formed in a parallel reaction and hydrogen peroxide—see U.S. Pat. No. 4,745,221—can lead to formation of byproducts that lower the peroxide selectivity, such as dihydroxylamine, nitrosyl, nitrous acid, nitric acid and nitrous oxide. It has also been observed that peroxide selectivity relative to the formed oxime decreases with increasing conversions of alkanones and cycloalkanones, and that complete conversions can be achieved only with simultaneously greater formation of byproducts.
Consequently, the content of water in the reaction mixture increases during the reaction. Since the solubility of long-chain and/or sterically bulky alkanones and cycloalkanones such as cyclooctanone and cyclododecanone as well as of their corresponding oximes decreases sharply with increasing water content in the reaction mixture, it is advisable to limit the quantity of water as much as possible during ammoximation. This is achieved by using hydrogen peroxide of the highest possible concentration in aqueous solution together with ammonia as dry gas.
In order to improve the conversion, it is of interest to minimize side reactions that produce water and lower peroxide selectivity, by choosing catalysts for application alone or if necessary in combination with cocatalysts.
As can be inferred from numerous documents, ammoximation is followed by distillative and/or extractive workup of the reaction mixture—see, for example, European Patents 0496385, 0208311, 0690045 and 0735017 as well as U.S. Pat. No. 4,794,198.
In a process having closed or partly closed solvent recycle, or in other words during continuous ammoximation, the water introduced and formed during the reaction must be additionally separated in an integrated or separate workup step, in order that the organic solvents present can be recycled to the ammoximation stage.
A disadvantage of distillative workup is that practically the entire solvent mixture must be distilled, thus consuming considerable energy and reducing the economy of the process.
Another disadvantage of reaction solutions containing higher molecular weight oximes is that separation of the carbonyl compound and its oxime from one another by distillation is incomplete or nonexistent. In such cases, therefore, it is of great interest to achieve the most complete ketone conversion possible, in order to be able to obtain oximes with the lowest possible carbonyl content.
In German Patent Application 10142620, there is described a method superior to distillative workup for separation of the oxime from the reaction mixture. The oxime is separated from the reaction mixture by crystallization, and the mother liquor is freed at least partly of water by subsequent pervaporation or vapor permeation by using at least one membrane separating stage. A disadvantage here is that, in the case of oximation of ketones with relatively high molecular weights under conditions of incomplete ketone conversion, the membranes can rapidly become fouled by ketone deposits and/or by deposits of ammonium salts present as cocatalysts, whereby expensive cleaning or even replacement of the membranes is necessary.